1. Field of the Invention
The present invention relates to an image reading device for reading images including characters, diagrams, figures and the like and converting the images to digital data. More particularly, the invention relates to an image reading device capable of adjusting digitizing conditions for an analog signal fed from an image sensor, such as a high reference voltage of an analog-to-digital converter which converts the analog signal to a digital signal.
2. Description of the Related Art
Image reading devices are incorporated into various equipments, such as facsimile devices. A conventional image reading device includes an LED array for irradiating light onto the surface of an original document to be read, and an image sensor for reading the image on the original document. The image sensor reads the image by picking up the light reflected from the surface of the original document. The image sensor outputs an analog signal having a voltage level determined depending on the density of the image. An analog-to-digital (A/D) converter is further included in the image reading device, which converts the analog signal to a digital signal.
In order to improve the tone gradation of the reproduced image particularly when a half-tone images are read with the image sensor, there has been proposed to adjust the high reference voltage of the A/D converter. This has been done by the provision of a voltage division circuit with which the analog signal fed from the image sensor is subjected to a voltage division. The resultant voltage divided signal is applied to the AID converter for digitizing the same. Based on the digital signal obtained from the A/D converter, the high reference voltage of the A/D converter is adjusted. However, there is a problem in the image reading devices with such a digitizing condition adjusting capability such that the provision of the voltage division circuit increases the manufacturing cost of the image reading device.
Further, image reading device provided with a filter circuit have been known in the art. The filter circuit is provided to enhance or emphasize the density of a picture element (pixel) depending on the density levels of the pixels adjacent to the pixel in question. By filtering the digital signal output from the A/C converter with the filter circuit, fine lines or low density lines can be reproduced. However, because the high reference voltage of the A/D converter is set to low when reading the original documents with high density background, the dynamic range of the analog signal input to the A/D converter is small and accordingly an S/N ratio of the digital signal output from the A/D converter is degraded. Filtering processing to such digital signal results in enhancing or emphasizing even noise components.
It has also proposed in the art to adjust the low reference voltage of the A/D converter which is a criteria for determining the A/D converted data corresponding to the black level image. To this effect, the conventional image reading device employs a Zenor diode to cut the d.c. component of the analog signal output from the image sensor. However, there are variations in the Zenor voltage caused by the variation in the Zenor diodes per se and also by the changes in the environmental temperature. In order to remedy the variation in the black level analog signal caused by the variation in the Zenor voltage, the low reference voltage of the A/D converter is adjusted to meet the black level analog signal actually applied to the A/D converter. Specifically, the digital signal representative of the black image is applied to an image processing circuit connected to the output of the A/D converter, and the output from the image processing circuit is converted to an analog signal by virtue of a digital-to-analog (D/A) converter and the resultant analog signal is used as the low reference voltage of the A/D converter. The D/A converter connected at its output to the low reference voltage terminal of the A/D converter must be of low output impedance so as to impedance match with the low reference voltage terminal of the A/D converter that is low in input impedance. That is, the available D/A converters for connection between the image processing circuit and the low reference voltage input terminal of the A/D converter must be low in output impedance. However, such A/D converters are expensive and so the use thereof is a cause of increase of the manufacturing cost.